Constant Interpolated Strings

C# 10 allows initializing const strings using string interpolation, but the placeholder must also be a const string.

The placeholder can't be another type, like numeric or date values, because they are sensitive to Culture, and can't be converted at compile time.

const string name = "Oleg";
const string greeting = $"Hello, {name}.";

Console.WriteLine(greeting);
// Output: Hello, Oleg.

Extended Property Patterns

Starting from C# 10, you can reference nested properties or fields within a proper pattern. The property pattern becomes more readable and requires fewer curly brackets.

Person person = new()
{
    Name = "Oleg",
    Location = new() { Country = "PL" }
};

if (person is { Name: "Oleg", Location.Country: "PL" })
{
    Console.WriteLine("It's me!");
}

class Person
{
    public string Name { get; set; }
    public Location Location { get; set; }
}

class Location
{
    public string Country { get; set; }
}

If Location is null, the pattern will not be matched and returns false.

File Scoped Namespaces

C# 10 introduces a new way of namespace declarations - file scoped namespaces.
However, you cannot declare a nested namespace or a second file-scoped namespace in the same file.

namespace FileScopedNamespace;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
    }
}

Global Usings

C# 10 adds a new modifier to a using directive - global. It means that using is applied to all files in the compilation.

All global using directives must be before non-global using directives. Global usings can be combined with a static modifier.

global using System;
global using System.Collections.Generic;
global using System.Linq;
global using System.Threading.Tasks;

List<int> list = new() { 1, 2, 3, 4 };
int sum = list.Sum();
Console.WriteLine(sum);

await Task.Delay(1000);

Assignment and Declaration in The Same Deconstruction

In previous versions of C#, a deconstruction could initialize newly declared variables or assign all values to existing variables.

C# 10 can do both assignment and declaration in the same deconstruction.

var rgb = (255, 100, 30);

// Initialization & assignment
int r;
(r, int g, int b) = rgb;

Console.WriteLine($"RGB: {r}, {g}, {b}");
// Output: RGB: 255, 100, 30

Sealed When Overriding ToString for Record Type

You couldn't add a sealed modifier when you override ToString method in the record type in C# 9.

In C# 10, you can do that. As in classes, it forbids to override ToString method in the derived record types.

Product product = new() { Name = "Bread" };
Console.WriteLine(product.ToString());
// Output: Bread

public record Product
{
    public string Name { get; init; }

    public sealed override string ToString()
    {
        return Name;
    }
}

Record Struct and Class Declarations

C# 10 introduces a record struct. You can declare it with record struct keywords. It also adds new syntax for a record class with record class keywords. The single record keyword from C# 9 means the declaration of record class. It was left for backward compatibility.

Person me = new() { FirstName = "Oleg", LastName = "Kyrylchuk" };

// 1. Built-in formatting for display
Console.WriteLine(me);
// Output: Person { FirstName = Oleg, LastName = Kyrylchuk }

// 2. Create a new record struct using with-expression
Person otherPerson = me with { FirstName = "John" };
Console.WriteLine(otherPerson);
// Output: Person { FirstName = John, LastName = Kyrylchuk }

// 3. Value equality
Person anotherMe = new() { FirstName = "Oleg", LastName = "Kyrylchuk" };
Console.WriteLine(me == anotherMe);
// Output: True

record struct Person
{
    public string FirstName { get; init; }
    public string LastName { get; init; }
}

// 4. You can create a positional record struct
record struct Product(string Name, decimal Price);

Parameterless Struct Constructors

Previous C# versions don't support field initializers in the structs. C# 10 fixes it and closes the gap between struct and class declarations. If the struct has field initializers, the compiler will synthesize a public parameterless constructor.

using System;

Person person = new() { Name = "Oleg" };

Console.WriteLine(person.Id + " " + person.Name);
// Output: 0cc6caac-d061-4f46-9301-c7cc2a012e47 Oleg

struct Person
{
    public Guid Id { get; init; } = Guid.NewGuid();
    public string Name { get; set; }
}

Lambdas with Attributes

C# 9 has allowed attributes on local functions. C# 10 allows attributes on lambda expressions and lambda parameters. To distinguish expression attribute from parameter attribute, you must use a parenthesized parameter list for lambda expression.

Action a =[MyAttribute] () => { };                          // [MyAttribute] lambda
Action<int> b =[return: MyAttribute] (x) => { };  // [MyAttribute] lambda
Action<int> c = ([MyAttribute] x) => { };             // [MyAttribute] x

class MyAttribute : Attribute
{ }

Explicit Return Type in Lambdas

In C# 10, you can specify an explicit return type for lambdas. Explicit return types are not supported for anonymous methods declared with delegate { } syntax.

Test<int>();

var l1 = string () => string.Empty;
var l2 = int () => 0;
var l3 = static void () => { };

void Test<T>()
{
    var l4 = T () => default;
}

AsyncMethodBuilder Attribute Applying to the Method

Since C# 7, you can apply the AsyncMethodBuilder attribute to a type only. In C# 10, you can also apply the attribute to a single method.

The attribute is read by the compiler and specifies that a type can be an async return type. Applying the attribute to the method you can specify a different async method builder for it. You can read more about it here and here.

using System.Runtime.CompilerServices;

class Example
{
    [AsyncMethodBuilder(typeof(AsyncVoidMethodBuilder))]
    public void ExampleMethod()
    {
    }
}

Expression With in Structs

C# 9 has introduced with expression for record types. It produces a copy of its operand with the specified properties and fields modified. In C# 10 you can also use with expression with struct types.

Product potato = new() { Name = "Potato", Category = "Vegetable" };
Console.WriteLine($"{potato.Name} {potato.Category}");
// Output: Potato Vegetable

Product tomato = potato with { Name = "Tomato" };
Console.WriteLine($"{tomato.Name} {tomato.Category}");
// Output: Tomato Vegetable

struct Product
{
    public string Name { get; set; }
    public string Category { get; set; }
}

Expression With in Anonymous Types

You can use with expression also with the anonymous types in C# 10.

var potato = new { Name = "Potato", Category = "Vegetable" };
Console.WriteLine($"{potato.Name} {potato.Category}");
// Output: Potato Vegetable

var onion = potato with { Name = "Onion" };
Console.WriteLine($"{onion.Name} {onion.Category}");
// Output: Onion Vegetable

Wrapping Up

All code samples you can find on my GitHub.